58-1 |
The mechanisms behind the shear effects on the crystallization of fats |
G. MAZZANTI1, S. E. Welch2, S. H. J. Idziak2, E. B. Sirota3, and A. G. Marangoni1. (1) Dept. of Food Science, Univ. of Guelph, Food Science Bldg., Guelph, ON N1G 2W1, Canada, (2) Dept. of Physics, Univ. of Waterloo, 332 Physics Bldg., Waterloo, ON N2L 3G1, Canada, (3) Corporate Strategic Research, ExxonMobil Research & Engineering Co., Route 22 E., Annandale, NJ 08801 Our original findings on shear induced phase transitions and crystalline orientation in the processing of fats prompted us to establish the mechanisms affecting the crystalline growth, phase transition and orientation of crystallizing fats. The present work describes the proposed mechanisms to explain these effects. The fats used were milk fat, milk fat triglycerides, palm oil and cocoa butter. The fats were crystallized from the melt in a temperature controlled Couette-Taylor cell. The diffraction patterns from Synchrotron X-Rays were captured on a 2D-CCD detector. The samples were cooled at controlled rates (3 and 0.5 °C/minute) down to the study temperature (17.5, 20 or 22.5 °C). The shear rates ranged from 45 to 2880 s-1. The segregation-agglomeration of platelet crystallites and the shear forces at the surface of already formed crystals are the two main mechanisms that explain shear effects. The acceleration of the alpha to beta-prime transition was increased with the shear rate in milk fat and palm oil. The increase was slow at low shear rates and became very strong above 360 s-1. In cocoa butter the acceleration between beta-prime-III and beta-V increased until a maximum of at 360 s-1, and then decreased, showing that competition between enhanced heat transfer and viscous heat generation are to be considered in the design of equipment. The orientation of crystallites was evident from low shear rates for milk fat and cocoa butter. Palm oil showed orientation above 180 s-1. The competition between Brownian and Shear forces, described by the Peclet number, determines the crystallites orientation. The critical radius size, from the Gibbs-Thomson equation, provides a tool to understand the effect of shear at the onset stages of crystallization. Our results offer a real possibility to understand and design dynamic crystallization processes from a sound scientific basis.
Session 58, Food Engineering: Transport processes and kinetics
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